Fluid treatment apparatus

ABSTRACT

Hollow fiber fluid fractionating apparatus in which the hollow fibers are twisted together to form cords which are assembled in a bundle in a fluid fractioning apparatus. The twists in the hollow fibers tend to break up boundary layers around the hollow fibers and improve the fractionating.

The present invention relates to fluid treatment apparatus which areformed with hollow fibres.

The fluid treatments considered in the present account are essentiallyseparation, exchange and mixing operations. As separation operations,there may be mentioned especially ultrafiltration, reverse osmosis, gaspermeation, gas diffusion and pervaporation. As exchange operations,there may be mentioned especially dialysis, direct osmosis, gas-gas,liquid-liquid or gas-liquid exchanges and heat exchanges; the artificalkidney involving dialysis falls into this category, as does theartifical lung. Finally, amongst mixing operations, there may bementioned especially dissolving gases in liquids, vaporisation ofliquids in gases and more particularly air conditioning (heat exchangeprocesses being involved) and humidification of gases and of air.

Very many apparatuses with hollow fibres have been proposed foreffecting the abovementioned treatments of fluids. For example, FrenchPat. Nos. 1,227,030, 1,307,979 and 1,340,495 may be mentioned. In allthese patents, the hollow fibres are either rectilinear over all, orwholly or partially incurved (this applies especially in the case ofbobbins).

In U.S. Pat. No. 3,616,928, it has been proposed to use wavy fibres,this makes it possible for the adjacent fibres not be parallel, at agiven point of the bundle of hollow fibres, because they have acurvilinear shape. This results in an improvement in the performance ofthe apparatuses, arising from the creation of turbulences inside andespcially outside the hollow fibres. These tubulences break up what arecalled the boundary layers of fluid which tend to form on either side ofthe dividing walls of the hollow fibres and which form an obstacle tocontinuing the operations of treating the fluid. In reverse osmosis andultrafiltration, what is involved is supersaturation upstream from thedividing wall while in dialysis, equalisation of the concentrations ofthe fluids on either side of the dialysis walls occurs. In every case,these boundary layers tend to cancel out the force gradient which playsa part in bring about the separation, exchange or mixing.

According to the present invention we provide an assembly of hollowfibres for use in a fluid fractionating apparatus, such assemblycomprising at least two hollow fibres of macromolecular material,twisted together in the form of a cord.

Such an assembly is simple to produce and the hollow fibres are in astable state, cannot be deformed easily, are arranged in a uniformmanner and are free from points where there are folds.

The invention also provides fluid fractionating apparatus comprising atleast one such assembly, a jacket enclosing the or each assembly, meansseparating the interiors of the various hollow fibres from the exteriorsof the hollow fibres within the jacket, at least one passage connectingthe interior of the jacket outside the hollow fibres to the exterior andat least one passage connecting the interior of the hollow fibres to theexterior of the jacket.

In operations involving exchange between fluids, the apparatusesaccording to the invention comprise one inlet passage and one outletpassage for each fluid (in general, two fluids). There are thusgenerally two inlet passages and two outlet passages (for casesinvolving two fluids).

In separation operations, there is generally at least one fluid flowinside the apparatus (which implies one fluid inlet passage and onefluid outlet passage). There are thus generally one inlet passage andtwo outlet passages.

In mixing operations, there are generally a first fluid and a secondfluid inside the apparatus, it being necessary to mix the second withthe first; the first fluid thus flows through the apparatus (whichimplies one inlet passage and one outlet passage) and there is an inletor a flow of the second fluid (which implies one inlet passage andoptionally an outlet passage). There are thus generally two inletpassages and at least one outlet passage.

The exact way in which the apparatuses of the invention are made up willnot be given in detail (except as relates to the cords of hollowfibres), because the structure of these apparatuses is not acharacteristic of the invention. The cords of hollow fibres arepreferably parallel and substantially rectilinear but they may or maynot be supported and can be in a coil or in a spiral, folded in theshaoe of a U or arranged perpendicular to one another.

It is understood that, in the present specification, the expression"hollow fibres" denotes fibres of tubular shape, that is to say fibreswhich contain within them a continuous channel which is positionedsubstantially along the axis of the fibre and is free frommacromolecular material.

The hollow fibres which can be used according to the invention can be ofany known type and can be made of any known natural, artificial orsynthetic macromolecular material; in particular, the fibres mentionedin French Pat. Nos. 1,307,969, 1,586,563 and 2,017,387 and U.S. Pat. No.3,674,628 may be involved; these fibres can be homogeneous ormicroporous or anisotropic (that is to say "with a skin"). They can beprepared by a melt method, by a dry method (evaporation of the solvent)or by a wet method (coagulation); the spinnerets employed are, inpractice, sector spinnerets or spinnerets which have annular orifices.The fibres which can be used according to the invention have an externaldiameter which is generally less than 1 mm and preferably less than 0.6mm, and generally greater than 5 μ and preferably greater than 300 μ.

The exact nature of the hollow fibres is chosen as a function of theapplication considered (osmosis, dialysis, ultrafiltration, gaspermeation, gas-gas or gas-liquid exchanges, heat exchanges and thelike).

A distinction will be drawn hereafter between the degree of twisting ofthe fibres relative to one another in a cord and the degree of twist inthe fibres themselves; the twist in the individual fibres is thedeformation which a hollow fibre undergoes under the effect of twoopposite torques acting in planes perpendicular to the axis of thefibres; the degree of twisting is the longitudinal deformation which ahollow fibre undergoes when the radius of curvature of the axis of thefibre is decreased, that is to say when this fibre is twisted.

The degree of twisting of the fibres in the invention is at least 5turns per meter (t/m), and is preferably between 30 and 300 t/m. Thedegree of twist of the individual fibres making up the invention can bepositive, zero or negative.

The cords of hollow fibres which can be used in the invention cancomprise any number whatsoever of individual hollow fibres; it is evenpossible to use cords in which one of the fibres is solid and has thefunction of increasing the strength of the cord. However, cords, arepreferred in which the number n of hollow fibres is less than 7, andthose in which n =2 are preferred still more.

According to another advantageous embodiment of the invention, so-calledcords "with no dead volume" are used. These so-called cords "with nodead volume" have the advantage of possessing practically no zone, inthe region outside the fibres, where fluids either could not flow orcould flow only very slowly.

It must also be stated that, in order to produce a cord with no deadvolume, a maximum degree of twisting exists for certain types of cordsand that this degree varies according to the nature and the exactdimensional characteristics of the hollow fibres employed. For a cord ofn hollow fibres twisted in an ordinary helix, this maximum degree oftwisting is achieved when the pitch of the helices is equal to n timesthe diameter of the fibres.

In order that the invention will be fully understood, the followingdescription is given, purely by way of example, reference being made tothe accompanying drawings, in which:

FIGS. 1, 2 and 10 are side elevations of three embodiments of hollowfibre assembly according to the invention;

FIGS. 3 to 9 and FIG. 12 are schematic views showing various ways offorming hollow fibre assemblies according to the invention; and

FIG. 11 is a perspective view of a fluid fractionating apparatusaccording to the invention.

A first type of hollow fibre assembly or cord is represented in FIG. 1.This is a cord comprising two hollow fibres twisted in the form of ahelix; the twisting can be right-handed or left-handed and the hollowfibres are in continuous and quasi-tangential contact. By"quasitangential contact" between two fibres, there is to be understoodthe situation of two fibres which, in a given cross-section, are incontact, only over less than 10 percent, and preferably less than 1percent, of their perimeter. When the degree of twist of the individualfibres is zero, the line of contact of the two hollow fibres ispractically a straight line.

A variant of the cord of FIG. 1 has been represented in FIG. 2. Here thehollow fibres are now in contact only at intervals.

A third type of hollow fibre assembly is shown in FIG. 10, in which fourhollow fibres are divided into two groups each of two fibres, to formtwo cords, the two fibres of each cord being twisted in a helix of agiven direction, and, moreover, the two thus formed cords are twistedtogether along a curve which also resembles a helix but is twisted inthe opposite direction to the helices of the two cords, thus forming adouble cord. The process D below describes the production of doublecords of this type.

In such a double cord, the information relating to the degree oftwisting given above (at least 5 t/m, and preferably 30 to 300 t/m)relates both to each cord of two hollow fibres and to each hollow fibrein a given cord considered on its own.

A double cord comprising four hollow fibres divided into two cords oftwo fibres has thus been represented in FIG. 10. However, it is equallypossible to produce a double cord comprising two cords each having nhollow fibres, the two cords forming a helix in a given direction andthe fibres in a given group substantially following a helix of thereverse direction.

The cords or double cords described above are advantageously combined inbundles of parallel cords. Improved uniformity in the flow of the fluidsis achieved when, in the said bundles, cords are juxtaposed in alternateorder, namely right-handed twisted cords, then left-handed twistedcords, then right-handed twisted cords and so on.

Another type of cord consists of hollow fibres which are practicallydevoid of torsional stresses, and this applies no matter what theirdegree of twisting may be. In such a cord at least one of the cordedhollow fibres has over at least a part of its length, a twist on itselfwhich is less than about one sixth of a revolution per turn. Such a cordcomprises several hollow fibres twisted together in a helix about acommon axis.

Twist free cords preferably consist of two hollow fibres twisted in ahelix, one around the other. The two hollow fibres are in tangentialcontact along a straight line which forms the axis of the cord and eachhollow fibre does not undergo any rotation on itself along a turn.However, in practice, minimum rotations of a few fractions of arevolution, in one direction or in the opposite direction, can bepermitted, and these fractions of a revolution can optionally offset oneanother. In general about one sixth of a revolution per turn, andpreferably about one twelfth of a revolution per turn, is not exceeded.

In the case of two hollow fibres which are twisted on one another andform two imbricated helices, if the external diameter of a hollow fibreis referred to as d, the pitch of the helix as p and the length of aturn as L, it is possible to define each of the helices forming the cordby the relationships: ##EQU1## noting that α is, on the one hand, theangle formed by a tangent to the helix and the line parallel to the axisof the cord drawn from the point of tangency, but is, on the other hand,the angle formed by the diagonal and the long side of a rectangle, thelong side of which is equal to the pitch and the short side to theperimeter of the base of the cylinder on which the helix is inscribed.

In order to make it possible to twist a cord with two hollow fibres thepitch p, cannot be less than the diameter d, and it is thus necessarythat: ##EQU2## that is to say ##EQU3## that is to say α<50°30 '

So that the hollow fibre is not subjected to any twist on itself whilstthe cord is being twisted, it must revolve on itself by 4α per turn inthe opposite direction to the twisting direction, α varying from 0 for ahelix of infinite pitch to 50°30 ' for the corresponding pitch P_(O) andtheoretically to 90° for a helix of zero pitch.

If two hollow fibres are twisted simply by twisting two parallel fibres,they each undergo a rotation on themselves of 360° per turn, thisrotation always being greater than that wich would be necessary to twistthem without torsional stress, since 4α<202°. The stress of twisting thehollow fibre on itself produced in this way increases whilst the pitchdecreases, because the number of revolutions per unit length increasesmore rapidly than the angle α. It is thus seen that if the degree oftwisting is increased (the pitch of the helix decreasing) it is thuspossible to flatten the hollow fibres. Thus, the cords of practicallytwistfree hollow fibres are very particularly suitable when the degreeof twisting of the hollow fibres is between 50 and 500 turns per meterand preferably between 150 and 300 turns per meter.

When the degree of twisting of the hollow fibres is high, thecross-section of a fibre through a plane perpendicular to the axis ofthe helices does not rigorously have the shape of an ellipse, but thatof a kidney bean with the long axis incurved.

In exchange apparatuses for effecting dialysis, for example, it isadvantageous that the hollow fibres should be in tangential contact onlywith a single other hollow fibre, or over a part of the length of thetwo hollow fibres, so as to prevent the formation of a dead volumebetween them. Thus, with this type of apparatus, cords consisting ofonly two hollow fibres, twisted with practically no twist in the fibresthemselves, are advantageously used.

With apparatuses for separating fluids, the dead spaces between fibrespossess fewer disadvantages than in exchange apparatuses, and cordsconsisting of three hollow fibres, twisted with practically no twist inthe fibres themselves are then preferably used.

Of course, it is possible for elementary cords of hollow fibres to becorded to one another with practically no twist. For example, it ispossible to cord together, without a twist in a positive direction,three cords each consisting of two hollow fibres which have been cordedbeforehand without a twist in a negative direction.

The twist-free cords of hollow fibres advantageously comprise hollowfibres twisted together with one and the same pitch, but they canoptionally comprise hollow fibres twisted with different pitches. Thehollow fibres can be corded over only a part of their length, but theyare advantageously corded over their entire length. The hollow fibres ofone and the same cord can be twisted with limited, advantageously equalbut sometimes different twists on themselves.

The hollow fibres of a cord according to the invention are subjected tostresses due to their twisting together with one another. However, ithas been found that these stresses are low relative to the stresses dueto any twist in the fibres themselves and that they hardly weaken anddeform the hollow fibres at all. Thus, for example, in a cord formed oftwo hollow fibres, when α = 18°, there is no more than 6 percentelongation from the outermost generatrix to the innermost generatrix.For the latter value, L = 10.76 d and p = 9.56 d. With d = 0.5 mm, atwist-free coil of 200 turns per meter is produced by revolving thefibres by 2π/5 on themselves in the reverse direction to the directionof twisting. The fibres are then not subjected to greater stress than ifthey were twisted with touching turns on a drum of diameter 160 mm.

It is also observed that the cords of hollow fibres with limited twistin the fibres themselves possess an increased elasticity which reducesthe risk of rupture. They make it possible to compensate for dimensionalvariations due to the uptake of moisture, the increase in the radiuscausing a shortening which substantially compensates for the axialelongation. It is also possible to reduce the thickness of the walls,and, for example in the case of haemodialysis, this makes it possible toreduce the dialytic resistance to small molecules.

The apparatuses and cords described above are especially advantageouswhen they are applied in artificial kidneys and/or lungs.

Some methods of twisting the fibres to form the cords will now bedescribed. In the figures relating to the descriptions of theseprocesses (FIGS. 3 to 9 and 12), the hollow fibres are represented by asingle line. The term bobbin is used to denote any support on whichfibres are wound in any way whatsoever. The initial bobbins of hollowfibres are generally those which can normally be obtained simply afterspinning.

METHOD A

According to this method, at least two fibres wound on at least onebobbin preferably turning on its axis are unwound axially and passthrough a yarn guide, the cord thus formed being preferably wound upsubstantially orthogonally to the axis of a cylindrical or conicalreceiving device such as a bobbin or drum. This process leads to cordssuch as those of FIGS. 1 and 2, but with a degree of twist which isgenerally not zero.

Different variants of this process A can be envisaged.

According to a variant A₁, represented in FIG. 3, two hollow fibres 2and 3, wound beforehand on a single bobbin 1 are unwound axially; thesetwo fibres pass through a yarn guide 4, and the cord 5 thus formed iswound on a bobbin 6, substantially perpendicular to the axis of the saidbobbin 6; a rotational movement is preferably imparted to the bobbin 1when it is desired to obtain a sufficiently high degree of twisting ofthe hollow fibres in the cord.

This method implies that, initially, two hollow fibres have been woundon one and the same bobbin. According to a variant, the startingcomponent is a bobbin having a winding of more than two hollow fibres,and this leads to a cord with more than two strands.

According to a variant A₂, shown in FIG. 4, two fibres 2 and 3, eachinitially wound on a different bobbin 1 and 1a, these bobbins beingcoaxial, are wound axially. This variant A₂ has the advantage, relativeto the variant A₁, of not requiring two different hollow fibres to bewound initially on one and the same bobbin 1.

According to a variant A₃, illustrated in FIG. 5, two hollow fibres 2and 3, each wound initially on a different bobbin 1 and 1a, are unwoundaxially as before, but in this particular case, the two bobbins 1 and 1aare not coaxial but are positioned coaxially with their axis ofrotation; these bobbins 1 and 1a are in fact carried by a support 7 towhich a turning movement is preferably imparted so as to provide thefibres with a sufficiently high degree of twisting. By imparting tothese bobbins a turning movement relative to the support 7, it ispossible to choose the degree of twist at will.

This method A₃ possesses substantially the same advantages as the methodA₂.

METHOD B

According to this method, at least two hollow fibres wound on at leastone fixed bobbin are unwound axially or non-axially, and then, afterpassing through a yarn guide, the cord of hollow fibres is wound upaxially on a receiving bobbin. This method, which leads to cords of thesame type as before, is represented in FIG. 6.

Two hollow fibres 2 and 3 are unwound axially from two fixed bobbins 1and 1a; after passing through a yarn guide 8, these hollow fibres arecarried along by passing over two rotating cylinders forming "a pair" 9;the two hollow fibres then pass successively through a fixed yarn guide10 and through a free running ring-and-traveller yarn guide 11 and arethen wound axially on the rotating receiving bobbin 12. The path of thetraveller 11 on the ring is represented by a broken line 13. A verticalreciprocating movement is imparted to the traveller 11, and this makesit possible to distribute the cord evenly over the bobbing 12 whichrotates on its axis in order to give a sufficiently large coil.

METHOD C

According to this method, which also leads to cords of the same type asabove, at least two hollow fibres, wound on a fixed bobbin, are unwoundaxially and are carried along by a rotating yarn guide, and then thecord formed is preferably wound up perpendicular to the axis of acylindrical (or conical) receiving bobbin.

According to a variant C₁, the rotating yarn guide is placed on arotating arm and the hollow fibre passes directly from the fixed bobbinto the yarn guide.

According to a variant C₂, represented in FIG. 7, the hollow fibres, atthe outlet of the fixed bobbin, pass through the axis of this samebobbin before gaining access to the rotating yarn guide. This methodmakes it possible to form the fibres into a cord with double the numberof turns with which the yarn guide rotated.

In FIG. 7, two fibres wound on a bobbin 1 are unwound axially, passthrough the yarn guide 14 firmly fixed to the support 15 of the bobbin1, through a channel provided in the rotating plate 16 and through ayarn guide 10, and the cord is then wound up on the receiving bobbin 6perpendicular to its axis. The bobbin 1 and its support 15 rest on anintermediate support 17 which itself rests on a ball bearing 18; in thisway, because of the inertia, the bobbin 1 is fixed during the rotationof the plate 16, secured to the ball bearing 18. The rotating plate 16thus acts as a rotating yarn guide.

METHOD D

According to this method, two helices with the same direction oftwisting and the same degree of twisting are imbricated into oneanother, these two helices individually consisting of a cord of n hollowfibres each, n being a positive integer, preferably less than 4. Onceimbricated into one another, the two helices have one and the same axisand are simply staggered relative to one another by half their pitch.

According to an embodiment D₁ of this method, two twisted hollow fibresor two cords each of n hollow fibres, kept twisted by a suitabletension, are placed side by side longitudinally, and then this tensionis relaxed, preferably whilst keeping the hollow fibres fixed togetherat one of their ends. Under these conditions, each of the twisted hollowfibres or cord of hollow fibres, which naturally have a tendency tountwist of their own accord, become twisted on one another during thisuntwisting process. The characteristic and advantage of such a two-helixcord, of 2 n hollow fibres in all, are its stability; the hollow fibrestherein are relaxed, that is to say twisted with practically no stress,so that the deformations of the cross-section of the fibres (flattening,collapse and the like) are reduced to a minimum; moreover, these cordscan be handled easily without becoming untwisted and without it beingnecessary to keep them fixed.

According to a variant D₂ of this method, a single hollow fibre or abundle of n parallel hollow fibres is twisted on itself; then, whilstkeeping this or these fibres fixed at their middle, their two ends arebrought close together until the two halves of the fibre aresubstantially rectilinear and are placed side by side against oneanother over their entire length. It is of course necessary to hold thefibres under tension in order to keep them substantially rectilinear andto prevent them from becoming untwisted. When this tension is relaxed,preferably by ceasing to fix the fibres at what was their middle, aself-coiling into a helix is observed, giving rise, as before, to a cordconsisting of two helices coiled on one another.

This variant D₂ has the advantage relative to the variant D₁ of leadingto more reliable and more reproducible results because the two helices,before imbrication, can be better held in position beside one another.

According to a variant D₃ of this method, it is possible to workcontinuously in the following way: two hollow fibres or two groups eachof n hollow fibres are twisted individually, by any means which is initself known, to the same degree (same number or turns/m), then the twocords thus produced are joined by a yarn guide, and then the whole iswound up axially on a receiving bobbin, the direction and the rate ofrotation of which are chosen so as to make it possible for the twoelementary cords to become somewhat untwisted, the mutual imbrication ofthe two helices taking place during this untwisting process. Of course,and preferably, the rate of rotation of the receiving bobbing is chosenso as to correspond precisely to the natural untwisting which the twoelementary cords would exhibit in a method such as D₂.

The method D₃ can, for example, be employed in an apparatus like thatwhich is described in FIG. 8 and which represents the manufacture of acord according to the invention comprising four hollow fibres.

Two bobbins 1 and 1b, each carrying a winding of two hollow fibres 2, 3and 2b, 3b respectively are unwound axially so as to form, after passingthrough the yarn guides 4 and 4b, two cords 5 and 5b, each consisting oftwo hollow fibres twisted in a helix in the same direction as eachother. These cords 5 and 5b are carried along by two pairs of rollers 9and 9b, and are placed side by side or combined by means of a yarn guide10b, and this causes them to become imbricated into one another so as toform the cord 19 after passing through a traveller 11b turning freely ona traveller 13; the rate of rotation of the receiving bobbin 12b ischosen so as to correspond to the natural untwisting which the cords 5and 5b would assume if they were imbricated into one another in a methodsuch as D₂.

METHOD E

This method leads to cords comprising a non-corded yarn and at least onehollow fibre twisted around. It differs from the method A, andespecially from its variants A₁ and A₂, by the fact that the non-cordedyarn is brought into the cords via the axis of the bobbins supportingthe hollow fibres and that, furthermore, it is possible to use only onebobbin with a hollow fibre.

This method is illustrated in FIG. 9. The non-corded yarn 21, initiallywound on a bobbin 20, is unwound perpendicular to the axis of thisbobbin and passes successively through the axes of the bobbin 1 and thenbobbin 1a. These bobbins are unwound axially: they are rotating in theopposite direction or preferably in the same direction, the rate ofrotation determining the degree of twisting of the fibres 2 and 3 whichunwind therefrom. These same fibres 2 and 3 are combined with thenon-corded yarn 21 by means of yarn guides 4 and 4a. The method iscarried out, in practice, so that the fibres 2 and 3 form non-touchinghelices. The cord 22 finally obtained is spread out, by means of theyarn guide 10a (to which a reciprocating movement is imparted), over thereceiving bobbin 6 where it winds up perpendicular to the axis.

METHOD F

It has been found that it is possible to produce twist-free cords ofhollow fibres if the hollow fibres are placed on bobbins which aredriven in the following way. Each bobbin describes an anti-planetarymovement produced by connecting it, for example, to a planet gearwheelof radius r which rolls without slipping on a fixed sun gearwheel ofradius R by means of a reversing planet gearwheel of any radiuswhatsoever.

Now, the planet gearwheel turns on itself, per complete revolution, by(1 - R/r) turn. In order to produce a twist-free cord of hollow fibres,the following relationship must apply: ##EQU4## α being determined bythe desired pitch and being expressed in degrees. The above relationshipmakes it possible to calculate the ratio R/r and, if, for example, α =18° is chosen with a fixed gearwheel of 80 teeth, it is found that theplanet gearwheel must have 100 teeth, it being possible for thereversing planet gearwheel to have any number of teeth whatsoever.

The hollow fibres are unwound simultaneously from the bobbins associatedwith the planet gearwheels through a yarn guide where the cord forms.The latter is wound on a drum after passing through a guide to which alateral reciprocating movement is imparted in order to ensure tht thecord is distributed evenly over the drum.

Of course, one and the same drum can receive several cordssimultaneously, for example two cords wound in the opposite direction.

The bobbins connected to the planet gearwheels describe circles, theaxis of which is identical to the axis of the sun gearwheel and canadvantageously be identical with the axis of the cord being formed.

It is preferred that the bobbins of hollow fibres connected to theplanet gearwheels should be cylindrical bobbins. It is also preferred tounwind the hollow fibres from these bobbins by pulling them offperpendicularly to the axis of the bobbins, or more precisely in anaverage direction perpendicular to their axis, the hollow fibre comingfrom a point travelling at any instant between the two ends of thebobbin. A cord thus formed from several hollow fibres is subjected topractically no torsional stress at any instant.

A cord is advantageously formed by twisting hollow fibres into helicesaccording to a constant pitch. To achieve this, it is possible to havethe rate of rotation of the drum which receives the cord controlled bythe speed of the rotor driving the unwinding bobbins. For example, it ispossible to have the motor driving the drum which receives the cordcontrolled by a tachometric dynamo or by an optical turn counter.Advantageously, these rates are kept constant in order to prevent suddenchanges in tension in the hollow fibres.

By way of example, a particular way for carrying out such a method hasbeen represented in FIG. 12, diagrammatically and not on any particularscale.

A system of anti-planetary gears is mounted in a casing 7 turning on ashaft 24. The casing 7 is driven by a motor 25 and a sun gearwheel 26,firmly attached to the shaft 24, is fixed. Two reversing gearwheels 27and 27' mesh with the sun gearwheel and two planet gearwheels 28 and 28'mesh with the reversing gearwheels and drive bobbins of hollow fibres 1and 1a contained in secondary casings 29 and 29'. The casings 29 and 29'are driven with a rotational movement by secondary shafts 30 and 30'connected to the planet gearwheels 28 and 28' by means of universaljoints 31 and 31', the shafts 30 and 30' being held in place by two arms23 and 23' attached to the casing 7. In operation, bobbins 1 and 1a areunwound perpendicular to their axes and the hollow fibres 2 and 3 rejoinone another at the yarn guide 4 where the cord 5 forms and is wound onthe drum 6 to which a reciprocating movement is imparted in order toensure that the cords are distributed correctly. A second cord 5a,coming from an apparatus identical to that already described, becomescoiled on the drum 6, which makes it possible to place on this drum ajuxtaposition of helices, the directions of which are alternatelyright-handed and left-handed. The drum 6 is driven by the motor 32 via abelt and electronic control means 33 have also been representeddiagrammatically, on the basis of the rate at which the cord 5 passes,as measured by the tachometer 34, make it possible to adjust the rate ofthe motors 25 and 32 and, where appropriate, to trigger an alarm 35. Aturning contact 36 enables one to check the rate of rotation of thecasing 7. Furthermore, attached motors 37 and 37' make it possibleoptionally to control the rotation of the bobbins 1 and 1a, the rate ofthe motors 37 and 37' being itself adjusted by turning contacts fixed tothe shafts 30 and 30'.

The following examples, which are given without implying a limitation,illustrate the invention and show how it can be carried out.

EXAMPLE 1 A - Production of the cord

By means of an apparatus like that of FIG. 3, a cord consisting of twohollow fibres is produced.

The hollow fibres are made of a copolymer of acrylonitrile and sodiummethallyl-sulphonate. Their external diameter is 522 μ and theirinternal diameter is 352 μ . Their wall is miocroporous so that, under arelative internal pressure of 0.5 bar, these fibres possess a flow rateto water of approximately 78 1/day m², the degree of rejection withrespect to ovalbumin (molecular weight = 45,000; used in the form of anaqueous solution of concentration 5 g/l containing 5.85 g/l of NaCl) is100 percent.

These fibres are twisted at the rate of 40 t/m.

B - Production of a dialysis apparatus

945 cords, prepared as indicated under A (and thus containing 1,890fibres) are placed parallel to one another so as to form a bundle ofparallelepiped shape, the dimensions of which are as follows:cross-section 1 cm × 8 cm, useful length: 41.6 cm.

An apparatus as illustrated in FIG. 11 is then made up. The bundle 100has end walls 101 of epoxy resin formed thereon, and it is secured in achannel section core 102, the epoxy resin end walls sealing against thewalls of the core to separate the interior of the hollow fibres from theexterior thereof within the core. The bundle of cores fits exactly intothe channel-shaped core. A lid 103 is secured to the open side of thecore and end caps 104, 105 close the ends, these end caps having inletand outlet pipes 106, 107 in communication with the interior of thehollow fibres, while pipes 108, 109 communicate with the exterior of thehollow fibres.

The useful surface area for dialytic exchange is 102.48 dm² (surfacearea calculated by attributing the average diameter equal to half of thesum of the internal and external diameters to the hollow fibres).

C - Use

A dialysis operation is carried out at 37°C for 6 hours using theapparatus produced under B.

A "product to be dialysed," containing the following constituents perliter, flows inside the hollow fibres via pipes 106, 107: 5.845 g ofNaCl, 0.224 g of KCl; 0.152 g of MgCl₂.6 H₂ O, 5.171 g of CH₃ COONa.3 H₂O, 0.33 g of CaCl₂.6 H₂ O and 1.5 g of urea.

Outside the hollow fibres there flows via pipes 108, 109, at a rate of500 cm³ /minute, a dialysate which is a saline solution which, at theinlet to the dialyser, has the same composition as that indicated forthe product to be dialysed, the dialysate being however free from urea.After use (a single pass), the dialysate is discharged from theapparatus.

A solution of "the product to be dialysed" passes through the hollowfibres at a flow rate of 200 cm³ /minute; it is conveyed to a graduatedvessel and then recycled to inside the hollow fibres; the graduatedvessel allows possible variations in the volume of the solution of theproduct to be dialysed to be detected, these variations being due to asecondary phenomenon of ultrafiltration through the walls of the hollowfibres; furthermore, when the volume of the product to be dialyseddecreases, an equivalent volume of new solution (having the compositionof the original product to be dialysed) is added so as to keep thequantity of the said product to be dialysed flowing inside the hollowfibres substantially constant.

Manometers make it possible to measure the pressures at the inlet and atthe outlet of the dialyser, in the case of both liquids flowing.

In the solution of the product to be dialysed, the pressure has beenadjusted to a value slightly higher than that of the dialysate outsidethe hollow fibres, this adjustment leading to an ultrafiltration of thesaid product to be dialysed of 500 cm³ /hour.

The clearance C of the urea is measured periodically. C is given by theformula: ##EQU5## in which: F is the flow rate of product to be dialysedinside the hollow fibres,

C_(i) is the concentration of urea at the inlet to the hollow fibres and

C_(o) is the concentration of urea at the outlet from the hollow fibres.

In fact, the clearance gives the nominal flow rate of product to bedialysed which would be obtained if the apparatus used provided aproduct to be dialysed totally freed from urea, without the rate ofremoval of urea being altered. The clearance thus defined does notcontain the fraction of urea removed by the ultrafiltration phenomenon.

During the experiment considered, a substantially constant clearanceequal to 125 cm³ /minute is observed for the urea.

Thereafter, the same experiment was carried out again, but with a flowrate of dialysate outside the hollow fibres of 1 l/minute; the clearancefor urea is then 139 cm³ /minute.

EXAMPLE 2

Example 1 is repeated, but with the following changes.

The solution of the product to be dialysed contains initially, asnon-saline solutes, 0.05 g/l of vitamin B₁₂ and 0.1 g/l of inulin.

The clearance for inulin and vitamin B₁₂ is defined as above for urea.

The flow rate of product to be dialysed outside the hollow fibres being500 cm³ minute, a clearance of 50 cm³ /minute is observed in the case ofvitamin B₁₂ and a clearance of 25 cm³ /minute is observed in the case ofinulin.

EXAMPLE 3 (comparison)

An apparatus with hollow fibres, of structure identical to that ofExample 1, paragraph B, is produced, but using non-corded parallelfibres. The number of fibres is 2,000, which corresponds to a usefulsurface area of 108.45 dm². The fibres are also identical to those ofExample 1.

Dialysis experiments are carried out under the same conditions as inExamples 1 and 2.

The flow rate of dialysate outside the hollow fibres being 500 cm³/minute, the following clearances are observed:

    urea             84 cm.sup.3 /minute,                                         creatinin        69 cm.sup.3 /minute,                                         vitamin B.sub.12 38 cm.sup.3 /minute, and                                     inulin           20 cm.sup.3 /minute.                                     

If these clearances are corrected in order to bring the useful surfacearea for dialytic exchange back to a value equal to that of Example 1,(values of) 80, 65, 36 and 19 are found respectively.

We claim:
 1. An assembly of hollow fibres for use in a fluid fractionating apparatus, said assembly comprising at least two hollow fibres of macromolecular material, twisted together in the form of a cord, wherein the degree of twisting of each individual fibre on itself is less than one sixth of a revolution per turn of twist of the fibres to form the cord.
 2. An assembly of hollow fibres as claimed in claim 1, wherein all the fibres of the assembly are twisted in the form of a cord.
 3. An assembly of hollow fibres as claimed in claim 1, wherein there are at most six hollow fibres twisted together to form a cord.
 4. An assembly of hollow fibres as claimed in claim 1, wherein the fibres are divided into two groups each consisting of n fibres, each group of n fibres being, in its entirety, twisted together in one direction in a helix to form a cord, the two cords being twisted with one another in the opposite direction in a curve similar to a helix to form a double cord.
 5. An assembly of hollow fibres as claimed in claim 1, wherein the degree of twisting of the fibres to form the cord is at least 5 t/m.
 6. An assembly of hollow fibres as claimed in claim 1, wherein the degree of twisting of each individual fibre on itself is substantially zero.
 7. An assembly of hollow fibres as claimed in claim 1, wherein the hollow fibres have an internal diameter of less than 1 mm.
 8. An assembly as claimed in claim 1, wherein the cords consist of two hollow fibres twisted in a helix, in continuous and quasi-tangential contact.
 9. Fluid fractionating apparatus comprising:a. at least one cord consisting of at least two hollow fibres of macromolecular material twisted together wherein the degree of twisting of each individual fibre on itself is less than one sixth of a revolution per turn of twist of fibre to form a cord; b. a jacket enclosing said at least one cord; c. means separating the interiors of the various hollow fibres from the exteriors of the hollow fibres within the jacket; d. at least one passage connecting the interior of the jacket outside the hollow fibres to the exterior; and e. at least one passage connecting the interior of the hollow fibres to the exterior of the jacket.
 10. Apparatus as claimed in claim 9, wherein the means for separating the interior of the hollow fibres from the exterior of the hollow fibres within the jacket comprise a hardened mass securing the at least one cord to the jacket.
 11. Apparatus as claimed in claim 9, wherein all the fibres are twisted in the form of a cord.
 12. Apparatus as claimed in claim 9, wherein there are at most six hollow fibres in each cord.
 13. Apparatus as claimed in claim 9, wherein the fibres are arranged in one or more assemblies, the fibres of an assembly being divided into two groups, each group consisting of n fibres, each group of n fibres being in its entirety twisted together in one direction in a helix to form a cord, the two cords being twisted with one another in opposite directions in a curve similar to a helix to form a double cord.
 14. Apparatus as claimed in claim 9, wherein the degree of twisting of the fibres to form the at least one cord is at least 5 t/m.
 15. Apparatus as claimed in claim 9, wherein the degree of twisting of each individual fibre on itself is substantially zero.
 16. Apparatus as claimed in claim 9, wherein the hollow fibres have an internal diameter of less than 1 millimeter. 